The accurate reproduction of patterns on the surface of a semiconductor substrate is critical to the proper fabrication of semiconductor devices. The semiconductor substrate may have undergone previous fabrication processes and may already feature layers and structures created by those fabrication processes. Improperly reproduced patterns can result in semiconductor devices that do not operate to design specifications or do not operate at all. For example, transistors can be created with improperly sized gates, conductors can be created that are short circuited or open circuited with other conductors or devices, structures can be created with wrong geometries, and so forth. Improperly reproduced patterns can reduce the yield of the fabrication process, thereby increasing the overall cost of the product. The reproduction process typically involves the use of optical lithography to reproduce the patterns onto the surface of the semiconductor substrate that is subsequently followed with a variety of processes to either subtract (for example, etch) and add (for example, deposit) materials from and to the semiconductor substrate.
However, as the dimensions of the structures making up the patterns continue to become smaller, their sizes approach (in some cases, the dimensions of the structures are smaller than) wavelength of the light used in optical lithography, the interference and processing effects can cause distortions and deviations in the patterns as they are reproduced onto the semiconductor substrate. In addition to the relationship between structures of the patterns and the wavelengths of the light, other factors that can cause distortion include the numerical aperture of the imaging system and the minimum pitch between structures in the pattern. The result being a reproduced pattern having a dramatically different appearance from the pattern being reproduced, also known as the intended pattern. The distortions and deviations in the reproduced pattern are dependent upon the characteristics of the pattern, such as the shape and size of the structures in the pattern, the presence of neighboring patterns and structures around the pattern, as well as the process conditions. For example, the interactions of the light with the structures making up a pattern can result in the reproduced pattern having rounded corners, bulges towards another elements, and so forth.
With reference now to FIGS. 1a and 1b, there are shown diagrams illustrating an exemplary pattern used in semiconductor device fabrication and a simulated reproduced pattern on a semiconductor substrate. The diagram shown in FIG. 1 a illustrates a pattern 100 that is to be reproduced on a semiconductor wafer. The pattern 100 includes a plurality of structures, such as structure 105, structure 106, structure 107, structure 108, and structure 109. Ideally, there will be a one-to-one correspondence between the pattern 100 and the reproduced pattern on the semiconductor substrate.
The diagram shown in FIG. 1b illustrates a simulation of the pattern 100 as it is reproduced onto the semiconductor substrate. For example, if a threshold photoresist model is used and the dose is set to a value of 3.3 times the dose-to-clear (i.e., the dose required to develop the resist in a large clear area), then intensities of greater than or equal 0.3 will print in the photoresist. These thresholds are shown in FIG. 1b. The diagram illustrates that the more isolated regions of the pattern 100 reproduce smaller, for example, pattern 130 and pattern 132, than the more nested regions, for example, pattern 134 and pattern 136.
Optical proximity correction (OPC) is a known technique wherein fragments of the structures making up the pattern can be modified (moved) so that associated mask patterns no longer look like the intended pattern, but through the previously discussed interactions between the light and the structures, the reproduced pattern on the semiconductor substrate made using the modified mask patterns will have an appearance that is closer to the intended pattern in appearance than the reproduced pattern made using the unmodified patterns. OPC is normally performed using computer-aided design (CAD) tools and involves the partitioning of edges of structures of a pattern into multiple fragments, which can be moved around to yield the desired reproduced pattern.
In some cases, however, conventional OPC techniques fail to adequately correct for pattern deviations because the required pattern modifications on the mask would violate mask design rules that define minimum line width and line spacing. This is particularly an issue with some resolution enhancement techniques using alternate phase shift masks (AltPSM) or attenuated phase shift masks where large deviations in the mask geometry could be required to affect small changes in the image geometry on the target wafer. What is needed are improvements to resolution enhancement techniques for reproducing patterns with a uniform width in cases where the use of conventional OPC techniques are impractical.